This invention relates generally to a system and method for communicating packetized data over a time division multiplexed communications system. In the early 1970""s, telephone began using a time division multiplexed (TDM) communications system, known as D4, that used a channel bank to multiplex and communicate time division multiplexed (TDM) voice signals over a communications link, such as a T1 link. The channel bank typically carried 24 digital voice signals between central telephone offices using only one pair of wires in each direction instead of the normal 24 pairs of wires required to communicate the 24 voice signals in analog form. This capability was achieved by digitizing and time division multiplexing the 24 analog voice signals into 24 channels or timeslots. In the TDM system, each of the channels is allocated a predetermined, equal amount of time (corresponding to a predetermined bandwidth) within each frame of the T1 link to communicate any data. Each channel is always allocated its predetermined amount of time, even if that channel has no voice data to transmit. In addition to communicating voice signals, these system may also communicate digital data because the D4 system was designed to handle digital data. The systems are still widely used today to carry voice traffic between central telephone offices. Therefore, the communications hardware and the network necessary for this D4 system are readily available.
A typical time division multiplexed (TDM) system, such as the D4 system, has a data rate of 1.544 million bit per second (Mbps) wherein timeslots of 64 Kbps are fixedly allocated to each channel unit. The 1.544 Mbps data rate is typically known as a T1 carrier.
Because conventional channel banks, such as the D4 system, have allocated fixed time slots for each channel, these systems suffer from an inefficient use of bandwidth and cannot dynamically allocate that bandwidth. For example, if one or more channels do not have any voice or data signals to transmit at a particular time, the timeslot assigned to that channel unit in the T1 frame is unused. In addition, if a particular channel has a need for more bandwidth than the allocated time slot, the TDM system does not allow that channel to request or receive any extra bandwidth. Due to these shortcomings, a number of alternative packet-based communications systems, such as asynchronous transfer mode (ATM), X.25 protocol, and frame relay, have been developed that do not assign fixed timeslots to each channel, but dynamically allocate bandwidth according to need. These packet-based communications systems are best used for digital data because digital data tends to be communicated in bursts. For example, a user sending a computer file that is 100 Kbytes long will need to send the entire 100 Kbytes as quickly as possible, but then will not require any more bandwidth until another transmission.
These packetized communications systems permit the total bandwidth of the communications link to be allocated in any manner depending on the need of the channels. For example, a single channel may use the entire bandwidth for several seconds because that channel has high priority digital data, such as an e-mail message or a computer file, that must be transmitted immediately. Most of the packetized communications systems provide some system for preventing any particular channel from over-using the communications link. These packetized systems, however, cannot use the hardware of an existing time division multiplexed channel bank, such as D4. Therefore, these packet-based systems require specialized hardware.
Thus, there is a need for a system and method for communicating packetized data over a channel bank, which avoids these and other problems of known devices, and it is to this end that the present invention is directed.
The invention provides a system for communicating packetized data over a channel bank previously used to communicate TDM data wherein the existing hardware of the TDM system may be used to communicate packetized data, which reduces the hardware cost of the packetized data communication system in accordance with the invention. In addition, the packetized data communications system in accordance with the invention may use existing data buses within the channel bank to communicate between the channel units. The invention also provides a system for communicating packetized data over the channel bank wherein a distributed contention system may be used such that each channel unit independently determines whether it has gained access to the channel bank. The packetized data communications system in accordance with the invention may have a system for preventing a channel unit from overusing the communications system.
In accordance with the invention, an apparatus for communicating packetized data over a communications link using a channel bank having a time, division multiplexing addressing scheme, the communications link having a predetermined format for data transmitted over the communications link including a framing bit and a frame period defined by the framing bit, wherein the apparatus comprises a plurality of channel units for generating packetized data, each piece of packetized data having a packet period corresponding to a sufficient amount of time to communicate the packetized data over the communications link, a system for allocating the communication link to a channel unit that is currently sending data over the communications link for the packet period, a system for skipping the framing bit when communicating said packetized data if the framing bit occurs during the packet period, a system for determining a next sender of packetized data over the communications link, the determining system being distributed over the plurality of channel units so that each channel unit independently determines whether it is the next sender, and a system for allocating the communications link to the channel unit that is the next sender of packetized data after said packet period so that the time division multiplexing addressing scheme of the channel bank is ignored, and the channel bank is communicating packetized data. A method for communicating packetized data over a channel bank is also disclosed.